1. Field of the Invention
The present invention relates to a contrast enhancement apparatus of a video signal and particularly, a contrast enhancement apparatus of a video signal, which performs processing contrast of a video signal at real time.
2. Description of the Background Art
Methods for improving picture quality of a video display apparatus comprise a method for making a user feel sharp by emphasizing the edge boundary and a method for making a user feel sharp by improving light and darkness contrast as a Homomorphic filter.
FIG. 1 is a block diagram showing a conventional contrast enhancement apparatus of a video signal using Homomorphic filter. As shown in the drawing, a contrast enhancement apparatus of a video signal comprises a logarithmic conversion unit 101 for converting a video signal into a logarithm, a wave filter 102 for separating the logarithm-converted video signals into a low frequency component and a high frequency component by the logarithmic conversion unit 101, a first multiplier 103A for attenuating the low frequency component into an attenuation coefficient, a second multiplier 103B for amplifying the high frequency component into an amplification coefficient, an adder 104 for adding the amplified high frequency component by the first multiplier 103A and attenuated low frequency component by the second multiplier 103B and an exponential conversion unit 105 for recovering the adding signal by the adder 104.
FIG. 2 shows a cumulative probability distribution function according to the number of pixels and luminance level of video signals. As shown in the drawing, in case of a dark portion of the video signal as reference numeral A of FIG. 2, the curve of cumulative probability distribution function between the number of pixels and luminance level is higher than a base dotted line. In case of a bright portion of the video signal as reference numeral B of FIG. 2, the curve of cumulative probability distribution function between the number of pixels and luminance level is lower than the base dotted line. In case of a intermediate portion of the video signal as reference numeral C of FIG. 2, the curve of cumulative probability distribution function between the number of pixels and luminance level is similar to the base dotted line. In case of a dark and bright portion of the video signals are mixed, as reference numeral D of FIG. 2, the curve of cumulative probability distribution function between the number of pixels and luminance level is lower or high than the base dotted line.
The conventional art will be described with reference to FIGS. 1 and 2.
A video signal is inputted to the logarithmic conversion unit 101 and the logarithm is converted and the logarithm-converted video signal is inputted to the wave filter 102 thus to be separated into a low frequency component and a high frequency component.
At this time, the first multiplier 103A multiplies an attenuation coefficient to the low frequency component and outputs the low frequency component attenuated as much as A times to improve contrast of the low frequency component. The second multiplier 103B multiplies the high frequency component by the amplification coefficient thus to output the high frequency amplified as much as B times.
The attenuated low frequency component and the amplified high frequency component are added in the adder 104 and the added signal is converted into an exponent through the exponential conversion unit 105 thus to be outputted recovered to an original video signal since the signal is a logarithm-converted signal.
The coefficients A and B are negative or positive according to the light and darkness and the size of the numbers means the degree.
Namely, the Homomorphic filter improves contrast of a video signal by attenuating an low frequency component and amplifying a high frequency component using a fixed amplifying or attenuating coefficient.
However, the conventional Homomorphic filter has a disadvantage that a video signal can not be treated adaptively since the low frequency component is attenuated and the high frequency component is amplified using a fixed coefficient.
Also, with the method of selecting a coefficient differently according to a video signal simply can not perform a real time processing and accordingly, contrast of the video signal can not be improved equally.
Also, the final output video signal should be scaled since the conversion phenomenon of a DC level occurs in case of attenuating the low frequency component and accordingly, a maximum and minimum value of the signal component should be obtained thus to cause a disadvantage in real time processing.
Therefore, an object of the present invention is to provide a contrast enhancement apparatus of a video signal which can provide an improved contrast at real time by generating the low frequency and high frequency mapping coefficients in a proper value according to the intrinsic property of the video signal, supplying the coefficient to the Homomorphic filter and controlling contrast of a video signal adaptively through first to fifth reference signals by a user.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a contrast enhancement apparatus of a video signal comprising a coefficient generation unit for generating a low frequency mapping coefficient which increases or attenuates the low frequency component of the video signal, an edge detection unit for detecting a rapid luminance changes in the vertical and horizontal direction, a logarithmic conversion unit for converting logarithm by receiving the inputted video signal, a low-pass filter for passing a low frequency component of the logarithm-converted signal, a high-pass filter for passing a high frequency component of the logarithm-converted signal, a first multiplier for multiplying a signal which passes the low frequency mapping coefficient and low-pass filter, a gain correction unit for adjusting the maximum value of the video signal which was increased or attenuated by the first multiplier to be in a certain level receiving the low frequency mapping coefficient outputted from the coefficient generation unit, a second multiplier for multiplying the signal passes the high-pass filter by the signal outputted from the edge detection unit, a first adder for adding the signals outputted from the first multiplier and the gain correction unit, a second adder for adding the signal outputted from the second multiplier and the first adder and an exponential conversion unit for converting an exponent of the signal outputted from the first adder.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.